Yoreh Deah Siman 18 Seif 3

 

ג צָרִיךְ לִבְדֹּק הַסַכִּין קֹדֶם, שְׁחִיטָה וְאִם לֹא בָּדַק, לֹא יִשְׁחֹט; וְאִם עָבָר וְלֹא בְּדָקָהּ תְּחִלָּה, וְאַחַר כָּךְ בְּדָקָהּ וּמְצָאָהּ יָפָה, שְׁחִיטָתוֹ כְּשֵׁרָה.

 

Seif 3

One needs to check his knife before (Slaughtering), slaughtering that is performed without first checking the knife, is not considered (proper) slaughtering, and if he transgresses and doesn’t check (the knife) at the outset, and if after (the slaughter) we check (the knife) and we find it suitable the slaughter is acceptable.